Skeletal muscle fibers possess two membrane systems which form the boundary between the inside of the cell and the external fluid: the surface membrane and the membrane of the transverse tubular system, a network which penetrates into the interior of the fiber and is involved in the mechanical activation of muscle. The methods described in this application provide a means for studying the permeability properties of the muscle membranes (using a voltage-clamp technique) and for localizing permeability changes as occurring on the surface or in the tubules. These techniques will be used first to study two of the permeability phenomena in muscle--anomalous rectification and delayed rectification. The evidence to date suggests that an early step in the mechanical activation of muscle involves depolarization of the tubular membranes. An attempt will be made to see whether a discrete movement of activating charge can be detected which results from this depolarization. Once the above studies have been completed in normal muscle, the same methods will be used to study muscles in certain abnormal conditions, particularly denervation. These experiments will enable us to determine the influence of innervation on the electrical properties of the surface and tubular membranes. Finally, it is planned to continue the investigation of the abnormal permeability properties of muscle fibers which are poisoned, then stimulated to exhaustion. These studies should provide information about the relationship between metabolic activity and membrane function. IDEN*